The present invention relates to positive electrode active material and lithium secondary batteries. More specifically, the present invention relates to lithium-deficient manganese layered composite oxide advantageous in cycle stability and capacity as compared to a conventional spinel type lithium manganese composite oxide, as positive electrode active material for rechargeable nonaqueous electrolyte secondary batteries, and nonaqueous electrolyte secondary batteries using, as positive electrode active material, such a lithium-deficient manganese layered composite oxide.
Among various rechargeable secondary batteries, the lithium secondary battery with its high charge-discharge voltage and large charge-discharge capacity has shown much promise as source of electricity for electric vehicles to meet the recent intense demand for zero-emission vehicles in environmental problem.
In consideration of some aspects of LiCoO2 such as the stability in operating environment, cost and natural reserves of LiCoO2 used as positive electrode active material for a lithium secondary battery, investigation is currently under way on spinel structure lithium manganese composite oxide (LiMn2O4) as positive electrode active material of a secondary battery for an automotive vehicle. Japanese Published Patent Applications, Publication (Kokai) Nos. 11(1999)-171550 and 11(1999)-73962 show spinel structure lithium manganese composite oxides as positive electrode active material of a secondary battery.
However, LiMn2O4 as positive electrode active material is deficient in durability at high temperatures and liable to cause deterioration in performance of the negative electrode due to dissolution of the positive electrode material into the electrolyte. To meet these problems, technique is on trial, of substituting various elements such as transition metals and typical metallic elements, for part of Mn. However, the partial substitution of various element for Mn to improve the high temperature cycle durability as disclosed in Published Japanese Patent Application, Publication (Kokai) No. 11(1999)-71115 is liable to cause distortion in the crystal structure and hence deteriorate the cycle durability at room temperature. Moreover, an increase in the amount of substitution for further improvement of the stability of the crystal structure tends to lower the capacity of the active material.
As to the capacity, lithium cobalt oxides (LiCoO2: the active material capacity=140 mAh/g) are higher in capacity than spine) type lithium manganese composite oxides (LiMn2O4: the active material capacity=100 mAh/g). However, lithium cobalt oxides are disadvantageous in the stability etc., as mentioned before. Therefore, a desired positive electrode active material is a high-capacity Mn containing lithium composite oxide which is higher in the Li content in the crystal structure than the spinel lithium manganese composite oxides (LiMn2O4) and which is superior in stability in operating environment to the lithium cobalt oxides.
In such a high-capacity type positive electrode active material for a lithium secondary battery, the lithium content in a chemical formula based on the crystal structure is determinative. Japanese Patent 2870741 seeks for a high-capacity Mn containing lithium composite oxide on the basis of crystal-chemical studies.
A recent report (A. Robert Armstrong and P. G. Bruce xe2x80x9cSynthesis of layered LiMnO2 as an electrode for rechargeable lithium batteriesxe2x80x9d, Nature, vol.381 (1996) p499) reveals LiMnO2 layered oxide has a positive electrode active material capacity of about 270 mAh/g, more than twice of that of a conventional spinel structure lithium manganese oxide.
With this layered oxide, a sufficient charge-discharge characteristic is obtainable at 55xc2x0 C., for example. However, the active material capacity decreases to about one third at room temperature. Moreover, the capacity is decreased gradually by repetition of charge and discharge at temperatures over room temperature, so that the cycle durability is insufficient.
It is therefore an object of the present invention to provide a lithium manganese layered composite oxide positive electrode active material which is higher in capacity than the conventional spinel structure lithium manganese composite oxide, and advantageous in high temperature cycle durability to the conventional layered structure lithium manganese composite oxide, and to provide a high-performance lithium secondary battery using this high-capacity lithium manganese layered composite oxide.
According to the present invention, a positive electrode active material for a nonaqueous electrolyte secondary battery, comprises: a lithium-containing manganese composite oxide having a layered crystal structure represented by the general formula LiMeO2. The lithium-containing manganese composite oxide is deficient in lithium with respect to a stoichiometric composition in the general formula LiMeO2, Me is a second constituent comprising Mn as a main component, and a substitute metal substituting for a part of Mn.
The lithium-containing manganese composite oxide is preferably represented by the formula Li1-xMn1-yMyOz where M is the substitute metal, x is a lithium deficiency quantity, y is a substitution quantity of the substitute metal substituting for a part of Mn, and z is an oxygen quantity z. The lithium deficiency quantity x is a rational number in the range of 0 less than x less than 1. The substitution quantity y is a rational number in the range of 0 less than y less than 1. The oxygen quantity z is equal to or smaller than 2. The oxygen quantity may be equal to 2, or may be smaller than 2 and greater than zero. The oxygen quantity z may be equal to 2-xcex4 (z=2-xcex4), and xcex4 may be in the range of xcex4xe2x89xa60.2.